Plasma arc torches employ a nozzle to constrain, direct, and control the plasma gas in order to control the arc of plasma gas generated by the torch.
FIG. 1 illustrates a typical example of a prior art nozzle 10. The nozzle 10 is symmetrically formed about a longitudinal nozzle axis 12, and has an orifice 14 that forms a passage through the nozzle 10 that is symmetrically formed about the nozzle axis 12. Typically, the orifice is generally cylindrical, and the nozzle 10 shown has a cylindrical orifice sidewall 16 that is stepped, having two cylindrical portions 18 and 20. The nozzle 10 also has a gas-directing surface 22 that is symmetrically disposed about the nozzle axis 12, and which extends normal thereto, joining to a nozzle interior sidewall 23 that is symmetrically disposed about the nozzle axis 12 and extends parallel to thereto, forming a cylinder centered on the nozzle axis 12. An orifice inlet 24 joins to the orifice sidewall 16 and to the gas-directing surface 22, and in the nozzle 10 is formed as a shallow cone centered on the nozzle axis 12. The inlet 24 is formed as a surface of rotation generated by rotating a line segment 26 about the nozzle axis 12, and the line segment 26 joins to the gas-directing surface 22 at an outer junction point 28, which defines an inlet diameter D, and joins to the orifice sidewall 16 at an inner junction point 30. The longitudinal distance of the inner junction point 30 from the plane in which the gas-directing surface 22 resides defines a nozzle depth Z. The nozzle 10 partially surrounds an electrode 32 having an emissive insert 34, and serves to control the flow of plasma gas that sustains the arc generated from the emissive insert 34.
In most cases, the plasma gas is introduced into the interior space of the nozzle 10 surrounding the electrode 32 (this space being partially defined by the nozzle interior sidewall 23 and the gas-directing surface 22) via a swirl ring (not shown) that directs the gas tangential to the nozzle interior sidewall 23 to form a swirling vortex. The gas-directing surface 22 serves to redirect the flow of plasma gas toward the orifice 14, and the orifice inlet 24 serves to transition the gas flow into the orifice 14, through which the gas passes. The conical orifice inlet 24 changes abruptly at the intersection with the orifice sidewall 16 at the inner junction point 30, this abrupt change tending to disturb the swirling gas flow.